1. Field of the Invention
The present invention relates generally to removing debris from waste flowing in a channel to a waste treatment facility, and more particularly to methods and apparatus for causing a large area travelling screen to pause and provide improved cleaning of the screen.
2. Description of the Related Art
Basins are used to remove certain undesired solid materials from liquid, for example, to clarify the liquid. For example, basins are used in water and waste water treatment plants, and in industrial waste water plants. Undesirable solid materials include non-settleable colloidal solid material that is mixed with chemicals to agglomerate and form solid materials that will settle from the liquid. In water and waste water treatment plants, undesirable solid materials include organic solids. In industrial processes, undesirable materials may include a variety of materials, such as fibers, which are not amenable to agglomeration and settling, for example.
Water, waste water, and liquid industrial wastes are treated in basins to remove such undesirable solid materials. The word xe2x80x9cliquidxe2x80x9d as used herein to describe the present invention refers to water, waste water, and liquid industrial wastes. These undesirable solid materials include particles that are suspended in the liquid. Some particles are removed from liquid by settling in low flow rate settlers, for example. Many undesirable solid materials are not removable by settling. For example, these include: (1) man-made materials such as plastics; (2) construction debris, containers and other large items that may be carried by the liquid but are not suspended in the liquid; and (3) many other materials (e.g., paper towels) that are carried by the liquid but do not degrade in the manner in which biodegradable materials (e.g., food) degrade.
For purposes of differentiating the undesirable, suspended solid materials from such other undesirable, non-settleable larger materials, these undesirable, suspended, settleable materials are referred to herein as xe2x80x9csolidsxe2x80x9d or xe2x80x9csettleable solidsxe2x80x9d, whereas these undesirable, non-settleable larger materials are referred to herein as xe2x80x9cscreenable solid materialsxe2x80x9d, or xe2x80x9cdebrisxe2x80x9d, because the debris are preferrably removed in xe2x80x9cscreeningxe2x80x9d equipment which permits the liquid and the settleable solids to flow past such equipment into the basin for settling of the settleable solids. Examples of such debris may include plastic products, paper, and dental floss.
If such debris are not removed before flowing into the basin, the debris may interfere with settling, debris in sludge from the settler may interfere with the removal of the sludge from the basin, and the sludge recovered from the basin may include some of the debris. The debris in the sludge is not desirable as it reduces the value of the sludge, and lessens the rate of settleable solids removed by settling per gallon of liquid.
Efforts have been made to improve ways of removing debris from liquids by screening. The removed debris may be referred to as xe2x80x9cscreeningsxe2x80x9d, for example. Fixed bar screens have worked effectively to remove debris in applications in which a channel for guiding a flow of the liquid and debris is not buried tens of feet under ground. For example, a prior art fixed bar screen offered for sale by Meurer Industries, Inc. has a front rake capable of cleaning the bar screen in a superior manner.
When the channel is buried tens of feet under ground, for example, too much time may be taken in moving the removed screenings up from the channel to the level of the floor of the treatment plant. Also, in many cases the stroke of a rake along the bars of the bar screen is too slow, and the extension of the rake arm down to the channel results in the rake arm extending many feet above the plant floor when the rake carries the removed debris up to the level of the floor. Therefore, although the efficiency of the basic screening operation of a bar screen may be acceptable, in certain applications the overall efficiency of bar screening is lacking as too slow or requiring too much height above the plant floor.
Others have attempted to improve screening by the use of so-called travelling screens. In these screens, an endless mesh screen is continuously moved into the liquid and the debris to gather the debris from the channel. As such screen continuously moves, the screen advances out of the liquid. In one type of travelling screen an endless screen continues to move out of the liquid and continue to move past a debris removal station at which the collected debris is brushed off such screen. See U.S. Pat. No. 5,387,336, for example. Alternatively, the collected debris may be removed from the screen by air or liquid, for example.
In one type of typical tooth-type travelling screen, the teeth are pivotally mounted and engage cams as the teeth continuously move. The cams cause one tooth to pivot and scrape against adjacent teeth in an attempt to remove the debris from the teeth. See U.S. Pat. No. 4,188,294, for example.
However, such blowing, brushing, teeth scraping, and other current attempts to remove the debris from the continuously moving travelling screens are not efficient enough. Thus, significant amounts of the debris stay on the travelling screen and move back into the channel. Unfortunately, the return path of such travelling screens is downstream of an upstream path that the liquid and the debris engage for removal of the debris. The debris is washed off this downstream path back into the liquid, which greatly lessens the efficiency of the travelling screen.
Further, even when the channel is near the ground, the area of a fixed bar screen through which the liquid flows is generally limited to the cross-sectional area of the channel, e.g., the channel width by the depth of the liquid and debris in the channel. Attempts to provide more screen area include providing a semicircular shape for each of a series of screen elements of a travelling screen. However, it is difficult to properly physically remove the debris from such semicircular screens, and the same problem of reintroducing the debris to the liquid downstream of the screen exists.
In view of the forgoing, what is needed is a way to efficiently remove the debris from a travelling screen so as to minimize the problem of reintroducing the debris to the liquid downstream of the screen. Further, the relatively high efficiency of fixed bar screens should be achieved while minimizing that problem.
Broadly speaking, the present invention fills these needs by more efficiently removing debris from a travelling screen so as to minimize the problem of reintroducing the debris to the liquid downstream of the screen. Further, while minimizing the problem the relatively high efficiency of fixed bar screens is achieved.
The present invention fills these needs by providing improved travelling screen elements for separating debris from liquid. Each of the screen elements has a filter surface through which the liquid but not the debris may flow. The filter surface is defined by a pair of planar plates, each of the plates having openings through which the liquid but not the debris may flow. A connector between the plates positions the plates relative to each other at a selected fixed angle. In one embodiment of the present invention, the selected fixed angle may be about ninety degrees. In this embodiment, the connector may be formed integrally with each of the plates. The integral version may be defined when the planar plates and the connector are formed from one sheet, and the connector is in the form of a bend to define the fixed angle and the two plates.
Provisions may also be made to mount one travelling screen element adjacent to another such element, and to move the adjacent elements into and out of the liquid and the debris in the channel. Each of the screen elements may have an angular cross section. One positioning of the series of elements is with some of the elements in the channel and relative to each other so that the angular cross sections cooperate to define a series of stairs for collecting debris from the channel. Another positioning of the series of elements is with some of the elements out of the channel and relative to each other so that the angular cross sections cooperate to change the stairs to define a substantially planar surface to facilitate removal of the collected debris from the elements.
The present invention also achieves these results by providing a travelling screen element assembly for receiving debris separated from liquid and transferring the travelling screen element assembly with the debris to a scraper. The assembly may include first and second screen elements. The first screen element has a filter surface defined by a first pair of planar plates, each of the plates of the first pair having openings through which the liquid but not the debris may flow. The first screen element also has a first connector between the plates of the first pair for positioning the plates of the first pair relative to each other at the selected fixed angle. The assembly may also have a second screen element having a filter surface defined by a second pair of planar plates, each of the plates of the second pair having openings through which the liquid but not the debris may flow. The second screen element may also have a second connector between the plates of the second pair for positioning the plates of the second pair relative to each other at the selected fixed angle. In the assembly, a guide positions the first and second screen elements adjacent to each other along a screen element scraper path with one planar plate of the first screen element being oriented relative to one planar plate of the second screen element so as to define a linear scraping path to facilitate removal of the debris from the screen elements by a scraper. In one embodiment of this assembly, the guide positions the first and second screen elements so that the linear scraping path has a first linear section defined by the one planar plate of the first screen element and a second linear section defined by the one planar plate of the second screen element. The guide positions the first linear section and the second linear section relative to each other at an angle of from about 175 degrees to about 195 degrees. In another embodiment of this assembly, the guide most preferrably positions the first linear section and the second linear section relative to each other at an angle of about 180 degrees so that the first linear section and the second linear section are aligned to define the linear scraping path as one plane, i.e., flat.
The present invention further achieves these results by having the guide position the first and second screen elements adjacent to each other along a debris collection path. The guide positions the first and second screen elements with one planar plate of each of the first and second screen elements oriented generally horizontally. As a result, these planar plates of the adjacent first and second screen elements define a series of generally horizontal steps for debris collection as the liquid and the debris flow toward the planar plates.
The present invention further achieves these results by having the guide position the first screen element and the second screen element with the respective planar plates extending out of and on one side of the debris separation path.
The present invention may further achieve these results by a method of positioning travelling screen elements for separating debris from the screen elements after the debris has been received on the screen elements within a flow channel. One operation of the method provides first and second screen elements according to the above description. Another operation mounts the first and second screen elements adjacent to each other along a debris separation path that is out of the channel so that the first screen element leads the second screen element along the path. One planar plate of the leading screen element is defined as a leading plate and the other planar plate of the leading element is defined as a trailing plate. One planar plate of the trailing screen element is defined as a leading plate and the other planar plate of the trailing element is defined as a trailing plate. With the first and second screen elements adjacent to each other along the debris separation path out of the channel, a final operation may be scraping the leading plate of the trailing element and the trailing plate of the leading element to separate the debris from these scraped plates of the screen elements. This scraping motion may be continuous, and the respective leading and trailing screen elements may be stopped during the scraping.
The present invention may further achieve these results by a method in which an operation provides a third screen element that is similar to the first and second elements. The first and second screen elements are moved to position the third screen element as a second trailing element that trails the first-recited trailing screen element. With the second and third screen elements adjacent to each other along the debris separation path, and out of the channel, and paused at a debris removal station, another operation scrapes the leading plate of the second trailing element and the trailing plate of the first trailing element to remove the debris from the leading plate of the second trailing element and the trailing plate of the first trailing element.
The present invention may additionally achieve these results by providing the above-described screen elements in a travelling screen for removing debris from liquid and debris flowing in a channel. In this travelling screen, there is an endless arrangement of such separate screen elements, each of the screen elements having two planar sections positioned relative to each other at the selected angle, e.g., at an angle about ninety degrees. A support is provided for the endless arrangement, the support positioning adjacent ones of the screen elements at a debris removal, or cleaning, station with the adjacent ones of the sections defining a planar surface. In this embodiment, each planar section of each element may be provided with perforations to allow the liquid to flow through the element while blocking the debris. The channel may have a cross section with a given area perpendicular to direction of the flow. The support may include spaced guides having a given width and spaced links located in the guides. The screen elements may extend between the guides to define a screen element width, the planar sections each having a substantially straight screen element length substantially greater than the first dimension. In implementing this embodiment, the spaced guides may be positioned in the channel at an angle relative to the direction of the flow through the channel.
The present invention may further achieve these results by a method in which debris is separated from liquid flowing in a channel. Operations of the method may include providing a series of the separate screen elements. The pairs of the screen elements are positioned adjacent to each other. A series of the elements is mounted with some of the elements in the channel and some of the elements out of the channel. The debris is collected on the screen elements that are in the channel. The screen elements are moved out of the channel to carry the collected debris out of the channel. The moving operation pauses with one pair of the adjacent elements positioned at the debris removal station. The paused pair defines a planar debris collection surface carrying the collected debris. In one embodiment, the planar debris collection surface is one planar section. In another embodiment, the planar debris collection surface is formed from two planar sections. A variation is that the two planar sections define two planes positioned at an obtuse angle that may be a peak or a depression. With the planar debris collection surface defined and carrying the collected debris, and the moving operation paused with one pair of the adjacent elements positioned at the debris removal station, a next operation is moving a scraper from a start position along the planar debris collection surface to scrape the debris off that planar surface. Once the scraping is complete, the series of elements is moved so that the previously-last screen element of the first pair and the first screen element of the next pair of screen elements define a next planar surface. This moving of the series of elements occurs while the scraper is moved to the start position.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.